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Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad Christian Wiebeler, Felix Plasser, Gordon J Hedley, Arvydas Ruseckas, Ifor D. W. Samuel, and Stefan Schumacher J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00089 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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The Journal of Physical Chemistry Letters

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad Christian Wiebeler,1, ∗ Felix Plasser,2 Gordon J. Hedley,3, 4 Arvydas Ruseckas,3 Ifor D. W. Samuel,3 and Stefan Schumacher1 1

Physics Department and Center for Optoelectronics and Photonics Paderborn (CeOPP), Universit¨ at Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany 2

Institute for Theoretical Chemistry,

Faculty of Chemistry, University of Vienna, W¨ ahringerstr. 17, 1090 Vienna, Austria 3

Organic Semiconductor Centre, SUPA,

School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, U.K. 4

Institut f¨ ur Experimentelle und Angewandte Physik,

Universit¨ at Regensburg, Universit¨ atsstrasse 31, 93053 Regensburg, Germany (Dated: February 15, 2017)

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Ultrafast relaxation dynamics following photoexcitation and electronic energy transfer (EET) in molecular systems are not only of fundamental interest1 but are of significance in light harvesting arrays,2,3 in OLEDs,4,5 and in organic photovoltaics.6–8 Förster theory, which requires the calculation of point- or line-dipole interactions, can be used to describe longrange EET.9,10 However, if donor and acceptor are too close, the theory does not hold.11–13 For specific cases, the rate of EET at short distances can be explained by hybrid approaches based on Redfield theory.14 In these approaches, the rate of EET is influenced by the spectral overlap and electronic coupling between donor and acceptor and the relative orientation of transition dipoles. Here we study EET in an orthogonal molecular dyad (BODT4).15 In this system with orthogonal orientation between acceptor and donor unit and strong electronic coupling, simplified models as discussed above are not applicable to understand the origin or characteristics of the EET following photoexcitation. In our experiments, the ultrafast molecular dynamics following excitation is analyzed based on time-resolved fluorescence depolarization, a powerful tool to understand relaxation processes in molecular systems.16,17 To gain true microscopic insight into dynamics and in particular EET, nonadiabatic molecular dynamics based on trajectory surface hopping can be employed.18 For large sized systems such dynamics require semiempirical methods like OM2/MRCI and DFTB for the quantum chemical calculations19,20 that are combined with the classical propagation of the nuclei in the excited states including nonadiabatic couplings allowing switches between different electronic states.21–23 For the system in our study, nonadiabatic ab initio molecular dynamics (NAAIMD) simulations are nowadays feasible.24–26 In our case the description of electronic states and excitations is firmly based on (time-dependent) density functional theory (TDDFT), where range-separated exchange-correlation functionals also accurately capture the nature of charge-separation over longer ranges. The molecular dyad under investigation (BODT4) consists of tetrathiophene (T4) as donor and a boron dipyromethene (BOD) dye as an acceptor. The primary photoexcitation of BODT4 mostly occurs on the T4 unit. Previous studies of conjugated oligomers indicate that following photoexcitation ultrafast relaxation and internal conversion occurs.28–31 In this case the classical path approximation32 may become inapplicable. A NA-AIMD approach was recently applied to oligothiophenes with a main focus on the ultrafast deactivation of short oligomers33,34 and on the role of hot charge-transfer states in a bithiophene 3




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depolarization experiments alone,15 these do not directly probe molecular structure or electronic states involved. Our theoretical calculations confirm the very efficient EET reported < 100 fs and reveal the involvement of an intermediate for BODT4 with transfer times of ∼ state with charge-transfer character in the transfer process. This notion may be important to inform future design and application of artificial systems with fast and efficient EET. The focus of the present study lies on the dynamical evolution of BODT4 following photo excitation. However, before we can move on analyzing dynamical properties, a solid understanding of the static electronic properties of the system needs to be obtained. In experiment, the absorption of BODT4 exhibits two prominent absorption peaks in the visible spectral range (cf. Ref. 15 and inset of FIG. 3). These features can be used to assess the reliability of the employed quantum chemical method for the computational description of the system. We find satisfactory agreement of the absorption spectra of the BODT4 molecule and its constituents, BOD and T4, calculated using (time dependent) density functional theory with the range separated CAM-B3LYP functional37–39 in connection with the 6-31G* basis set using Gaussian 09.40 This functional is known to provide an accurate description of extended π-conjugated systems41–44 . Molecular structures and calculated spectra are shown in FIG. 1. Optimization of the BODT4 geometry results in a structure, where the plane of the thiophene ring of T4 that is the closest to BOD is nearly perpendicular to the corresponding plane of BOD. With the range-separated functional used, we obtain two distinct absorption peaks in the range from 300 to 500 nm, which are both blue-shifted relative to experiment, compare FIG. 1 (b) with the inset in FIG. 3. Comparing the absorption of BODT4 with the absorption of its building blocks, the lowest energy peak can be attributed to absorption of BOD and the second peak is traced back to absorption of T4. These findings are consistent with the previously reported study on BODT4.15 The calculated excited states can be further characterized based on natural transition orbitals (NTO).45 We calculate NTOs for each transition of interest using TheoDORE.46,47 They describe an electronic excitation with a minimum number of effective single-particle orbitals. In our case each investigated transition is dominated by one pair of orbitals, see FIG. 1. So one of the orbitals can be interpreted as the orbital of the hole and the other one as the orbital of the excited electron. The lowest energy excitation of BODT4 is mainly localized at BOD, its excitation energy is slightly red-shifted compared to the lowest energy excitation of BOD and only small differences in the oscillator strengths are found. Also 5




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with the experiment in FIG. 3 (inset) is found based on spectrum simulation via nuclearensemble approach (NEA)48 interfacing Newton-X49 and Gaussian 09.40 This approach takes the quantum mechanical zero point vibrations in the electronic ground state into account. It can be employed to model absorption bands more accurately than single-point calculations (though vibronic transitions are not explicitly considered). A vibrational analysis for the electronic ground state is used to generate an ensemble of 1000 geometries based on a Wigner-distribution. Subsequently, the first ten excited states for each geometry are calculated. The resulting ensemble spectrum is shown in FIG. 2. The absorption of T4 in the plotted range is dominated by the absorption of the S1 state. This state has a strong variation in excitation energies depending on the actual geometry, leading to a broad absorption band for the ensemble. The ensemble absorption into the S1 state of BOD is significantly narrower. The absolute absorption of T4 is in good agreement with experiment, the absolute absorption for BOD is slightly too low compared to experiment as the broadening is slightly overestimated.15 Overall, the absorption for the two peaks of BODT4 is dominated by the absorption of the corresponding building blocks. It remains similar in shape and is slightly red-shifted. In the following we turn our attention to the ultrafast dynamics of photoluminescence (PL) emitted by BODT4 after photoexcitation. Experimental results are shown in FIG. 3 and were recorded using the fluorescence upconversion technique described previously.50 A low concentration solution of BODT4 in cyclohexane was placed in a rotating sample holder to minimize laser degradation. The sample was excited by light pulses of 100 fs duration at a wavelength of 425 nm and repetition rate of 80 MHz by frequency doubling the output of a Ti:Sapphire laser operating at 850 nm. This ensured optical excitation of T4 with almost no excitation of any higher absorption bands of BOD. The excitation window is on the red side of the main higher-energy absorption feature. The residual 850 nm light acted as the gating pulse and was mixed with 600 nm PL from the sample in a BBO crystal to produce upconverted light at 350 nm that was spectrally filtered from all other wavelengths and detected on a photomultiplier tube. Scanning of an optical delay line on the gating pulse enabled temporal resolution to be obtained. The instrument response function, obtained by upconverting Raman signals from water, gave a 270 fs full-width half-maximum Gaussian profile. 600 nm was chosen as the detection wavelength to ensure that only BOD PL was recorded and that there was no overlapping PL from the initially accessed T4. The obtained 7




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First we turn our attention to the average population of excited states following photoexcitation. The dynamics of these (classical) populations are shown in FIG. 4. A pronounced decay of the states S2 to S4 within the first 100 fs is found and the S1 state is mainly populated. This latter state is taken as an indicator that an EET to the BOD part has taken place, so that its rise time corresponds to the rise time of the signal measured in experiment. This assumption is based on the static calculations where the S1 state is localized at BOD. However the actual character of this excited state might depend on the geometry as discussed in more detail for a sample trajectory below. After the relaxation into the S1 state, in the microcanonical ensemble without thermostat or explicit solvation, a dynamical equilibrium is reached for the excited state manifold, showing only small fluctuations of the populations of individual states. The decay of the sum of the populations of the states S2 to S4 and the rise of the S1 state population were fitted using exponential functions with an offset of 0.2 to account for the residual population found in the higher excited states. From both fits a time constant of 62 fs was obtained. This reproduces fairly well the experimental timescale for fluorescence de-polarization of 120 fs for BODT4 in cyclohexane solution.15 We note that the prediction of actual time constants is known to be quite sensitive to the method employed for electronic structure calculations.33 Furthermore, no coupling to the environment was included. No removal of excess energy nor interaction with surrounding solvent molecules occurs in the calculations. Both is expected to slow down the EET dynamics. We can also determine the orientation between the transition dipole of the excitation and at the end of the dynamics simulations, i.e. at 300 fs, for the current states of the trajectories. We find an average angle of 81 ◦ which is also in fair agreement with the experimental value of 64 ◦ . Having obtained first insights into ensemble averages and average time constants for excitation energy transfer above, the fully microscopic approach used here allows a much more detailed understanding of the dynamics and underlying processes for individual molecules. In particular analyzing the character of the electronic states involved in the dynamics will give detailed information about relevant microscopic transfer mechanisms. Therefore, we further investigate a representative trajectory showing ultrafast relaxation and EET. We note that FIG. 5 shows a particularly fast relaxation and EET but is representative for a large number of trajectories in the main features of the transfer process as discussed below. In FIG. 5 the character of the current electronic state is tracked over time by depicting excitation energy and oscillator strength. The starting point of the trajectory is an excitation 10


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the oscillator strength to nearly zero as found in 80 % of the computed trajectories. Upon relaxation into the first excited state, electron and hole are still separated in space. After about 10 fs of dynamics in the hot S1 state, S1 and S2 are still close in energy and population is repeatedly transferred between the two states, cf. FIG. S9. Afterwards, the S1 state localizes at BOD and after about 20 fs, the characteristics of the current excited state are similar to the first excited state obtained for BODT4 reported in FIG. 1, i.e. excitation energy between 2.5 and 3.0 eV and an oscillator strength of about 0.5. These two typical features are found at the end of 60 % of all computed trajectories. For the remaining trajectories, similar behavior is still found but is accompanied by stronger fluctuations of the oscillator strength and hoppings back into higher excited states. The latter could potentially be eliminated dissipating excess energy to the environment. We find no signature of a hot excited state on the BOD unit during or after the EET, in agreement with conclusions drawn from experiment.15 In contrast to this, the relaxation through the excited states is mediated by a state with charge-transfer character. We note that the energy alone is not an appropriate measure for the character of an excited state, because the excitation energies strongly depend on the molecular geometry and the energetic ordering of excitations changes during the dynamics. We observe that for the sample trajectory shown, first relaxation into the first excited state occurs, directly followed by transfer of the excitation. As a similar behavior is found for many trajectories, fitting the rise of the S1 state population might lead to a slight underestimation of the actual EET time. To characterize the temporal evolution of the excited states further, the position of the excitation47 and the size of the corresponding exciton56 can be investigated. These results are reported in FIG. S10 of the Supporting Information for the sample trajectory discussed. This figure highlights the charge-transfer character of the intermediate state but also shows that the charge separation is never complete, i.e. there is a strong mixing between local and charge-transfer states. We note that discussion and nomenclature of states on such fast timescales can be challenging, and thus the mixing of local and charge transfer states will lead to a complex picture not easily characterized by simple terminology, such as calling a state ”charge transfer”. None the less, the calculations clearly reveal that purely local states cannot account for the EET observed. In this study, we investigated the absorption and excitation energy transfer mechanisms in BODT4 on a microscopic level using state-of-the-art quantum chemical methods. Our investigations demonstrate that the different broadenings for the two absorption peaks of 12


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BODT4 can be traced back to the absorption of the corresponding building blocks and the broad structure of the second peak is caused by the structural flexibility of the T4 part of the BODT4 molecule. Based on non adiabatic ab-initio molecular dynamics (NA-AIMD) simulations, we find that ultrafast relaxation and excitation energy transfer occurs on a time scale of 62 fs, in fair agreement with experimental findings based on fluorescence depolarization. On a microscopic level, we find that EET is mediated by an intermediate state with charge-transfer character after ultrafast relaxation of the initial excitation on the T4 unit. The important role a charge-transfer state plays in the ultrafast EET observed in the present study could also inform attempts realizing artificial systems showing efficient EET. The same theoretical approach used here would also allow detailed studies on EET in related molecular dyads of slightly larger size,57 and of further ultrafast photophysical processes in molecular dyads that are for example composed of BOD as fluorophore and of diarylethenes as photoswitch.58,59 This could elucidate the role of different underlying light-induced processes on a molecular level.

Associated content

Supporting Information Description and further information regarding the methods for spectrum calculation (Section A), spectrum simulation using the nuclear-ensemble approach (Section B), and nonadiabatic dynamics simulations (Section C); further analysis of the trajectory shown (Section D), and a full author list of Reference [40] (Section E). An animation of the trajectory analyzed in FIG. 5 is included in the online material.

Author information

Corresponding author [email protected] Notes The authors declare no competing financial interest.

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Acknowledgements

C.W. and S.S. thank the German Research Foundation (DFG: GRK 1464) for financial support and PC2 for computing time. S.S. acknowledges support through the Heisenberg programme of the DFG. F.P. acknowledges support by the VSC Research Center funded by the Austrian Federal Ministry of Science, Research, and Economy (bmwfw). The St Andrews group acknowledges support from the European Research Council (grant number 321305) and the Engineering and Physical Sciences Research Council (grant EP/L017008/1). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award.

∗

Present address: Fritz Haber Center for Molecular Dynamics Research, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

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Ultrafast Electronic Energy Transfer in an ... - ACS Publications

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